Compound solar and manufacturing method thereof

ABSTRACT

On a surface of a GaAs substrate, layers to be a top cell are formed by epitaxial growth. On the top cell, layers to be a bottom cell are formed. Thereafter, on a surface of the bottom cell, a back surface electrode is formed. Thereafter, a glass plate is adhered to the back surface electrode by wax. Then, the GaAs substrate supported by the glass plate is dipped in an alkali solution, whereby the GaAs substrate is removed. Thereafter, a surface electrode is formed on the top cell. Finally the glass plate is separated from the back surface electrode. In this manner, a compound solar battery that improves efficiency of conversion to electric energy can be obtained.

This nonprovisional application is based on Japanese Patent ApplicationsNos. 2003-115360 and 2003-123328 filed with the Japan Patent Office onApr. 21, 2003 and Apr. 28, 2003, respectively, the entire contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a compound solar battery andmanufacturing method thereof and, more specifically, to a multi-junctiontype compound solar battery and manufacturing method thereof.

2. Description of the Background Art

A multi-junction type III-V group compound solar battery has been knownas a solar battery having highest efficiency and most suitable foraerospace applications among solar batteries. An exemplary method ofmanufacturing such a multi-junction type III-V group compound solarbattery will be described in the following.

First, referring to FIG. 34, a Ge substrate (or a GaAs substrate) 101 isused as a substrate. On a surface of the substrate 101 of Ge or thelike, Ge is epitaxially grown and AsH₃ or PH₃ is added to cause thermaldiffusion of As or P, so that a bottom cell BB including a pn-junctionof Ge is formed.

On the bottom cell BB, GaAs is epitaxially grown, so that a middle cellMM including a pn-junction of GaAs is formed. On the middle cell MM,InGaP is epitaxially grown, so that a top cell TT including apn-junction of InGaP is formed.

In this manner, a 3-junction type group compound solar battery 110having a cell body CC is formed, in which three pn-junctions ofGe/GaAs/InGaP are connected in series in this order from the lower sideon Ge substrate 101.

Forbidden band width (band gap) of InGaP forming the top cell TT isabout 1.7 to about 2.1 eV, that of GaAs as the middle cell is about 1.3to about 1.6 eV, and that of Ge as the bottom cell is about 0.7 eV orlower.

Sunlight enters from the side of top cell TT (InGaP) and proceeds towardthe bottom cell BB (Ge), while light of prescribed wavelength isabsorbed in accordance with the band gap of each of the top cell TT,middle cell MM and bottom cell BB, to be converted to electric energy.

Here, the value of the band gap (about 0.7 eV or lower) of Ge as thebottom cell is relatively small considering the function of convertingoptical energy to electric energy. Therefore, use of a material havingband gap of about 0.9 to about 1.1 eV has been proposed, as a materialhaving higher conversion efficiency.

Reference 1 (M. Tamura et al., “Threading dislocations inIn_(x)Ga_(1-x)As/GaAs heterostructures”, J. Appl. Phys. 72(8), Oct. 15,1992, p. 3398) proposes InGaAs as one such material. In a multi junctiontype solar battery 110 using InGaAs in place of Ge, on Ge substrate (orGaAs substrate) 101, a bottom cell NN including a pn-junction of InGaAsis formed by epitaxial growth.

On the bottom cell NN, the middle cell MM including the pn-junction ofGaAs and the top cell TT including the pn-junction of InGaP would beformed by epitaxial growth, respectively.

Reference 2 (J. F. Geisz et al., “Photocurrent of 1 eV GaInNAslattice-matched to GaAs”, J. Crystal Growth 195 (1998), p. 401)proposes, in addition to InGaAs, InGaAsN as a material to replace Ge.

Multi-junction type solar battery 110 having the bottom cell NNemploying InGaAs or InGaAsN in place of Ge, however, has the followingproblems.

First, in a multi-junction type solar battery employing InGaAs (0.9 to1.1 eV) as the bottom cell NN, lattice constant of Ge substrate (or GaAssubstrate) 101 is different from that of InGaAs. Therefore, epitaxiallygrown InGaAs comes to have a dislocation derived from the difference inlattice constant from the underlying layer (GaAs substrate or the like)(hereinafter referred to as a “misfit dislocation”).

In the multi junction type solar battery employing InGaAsN as the bottomcell, composition of N atoms will be controlled such that the latticeconstant of InGaAsN matches the lattice constant of the underlyinglayer. Therefore, generation of misfit dislocation can be prevented inthe epitaxially grown InGaAsN.

It is noted, however, that there would be holes and the like of added Natoms themselves. As a result, the epitaxially grown InGaAsN comes tohave defects derived from N atoms.

As described above, the bottom cell formed of InGaAs or InGaAsN suffersfrom generation of misfit dislocation or defects, and therefore it doesnot have satisfactory cell quality. Accordingly, desired electricityproduction cannot be attained.

Further, the misfit dislocation or defects in the bottom cell NN hasundesirable influence on GaAs as the middle cell MM epitaxially formedon the bottom cell NN and on InGaP as the top cell TT further formedthereon.

Consequently, cell quality of GaAs and InGaP is also degraded,preventing improvement in efficiency of electric energy conversion.

As described above, sunlight enters from the top cell TT and proceeds tobottom cell BB while light of a prescribed wavelength is absorbed andconverted to electric energy.

At this time, component of the sunlight that is not absorbed by the topcell TT to bottom cell BB is eventually absorbed by Ge substrate (orGaAs substrate) 101 and hence that component cannot effectivelycontribute to generation of power.

As a result, improvement in efficiency of electric energy conversion isprevented.

SUMMARY OF THE INVENTION

The present invention was made in order to solve the above describedproblems, and an object is to provide a compound solar battery that canimprove efficiency of conversion to electric energy, and another objectis to provide a method of manufacturing such a compound solar battery.

According to an aspect, the present invention provides a compound solarbattery including a cell body, a first electrode portion and a secondelectrode portion. The cell body has at least one pn-junction layer ofsingle crystal, and sunlight enters thereto. The first electrode portionis formed directly on that surface of the cell body which is opposite tothe sunlight entering side, and has a prescribed thickness to supportthe cell body. The second electrode portion is formed on a surface ofsunlight entering side of the cell body.

In this structure, the first electrode portion is formed directly onthat side of the cell body which is opposite to the sunlight enteringside. Thus, different from the conventional structure in which aprescribed substrate is arranged for epitaxial growth on that side ofthe cell body which is opposite to the sunlight entering side, thecomponent of sunlight that enters the cell body but not absorbed by thecell body is reflected by the first electrode portion. As a result, theeffect of light confinement improves, and the conversion efficiency ofthe compound solar battery can be improved.

Preferably, the cell body and the first electrode portion are flexible,or the first electrode portion preferably has such a thickness thatallows deflection.

Accordingly, the compound solar battery comes to have higher degree offreedom in its shape.

As to more specific structure of the body, preferably the cell bodyincludes a plurality of pn-junction layers of compounds having mutuallydifferent band gaps, and preferably, the plurality of pn-junctions arearranged such that the band gaps are made higher from the side of thefirst electrode portion to the sunlight entering side.

More specifically, the plurality of pn-junction layers preferablyinclude a first pn-junction layer of a III-V group compound having afirst band gap formed on the first electrode portion and a secondpn-junction of a III-III-V group compound having a second band gaphigher than the first band gap formed on the first pn-junction layer.

Alternatively, the plurality of pn-junction layers preferably include afirst pn-junction layer of a III-III-V group compound having a firstband gap formed on the first electrode portion, a second pn-junctionlayer of a III-V group compound having a second band gap higher than thefirst band gap formed on the first pn-junction layer, and a thirdpn-junction layer of a III-III-V group compound having a third band gaphigher than the second band gap formed on the second pn-junction layer.

Alternatively, the plurality of pn-junction layers preferably include afirst pn-junction layer containing a I-III-VI group compound having afirst band gap formed on the first electrode portion, a secondpn-junction layer of a III-III-V group compound having a second band gaphigher than the first band gap formed on the first pn-junction layer,and a third pn-junction layer of III-III-V group compound having a thirdband gap higher than the second band gap formed on the secondpn-junction layer. Here, I, III, V and VI groups represent groups of theperiodic table.

Further, besides the cell body, another cell body may be providedadhered on that side of the first electrode portion which is opposite tothe sunlight entering side, and in that case, the first electrodeportion is preferably formed of a transparent conductive film.

More specifically, the cell body preferably has pn-junction layersarranged such that the band gap becomes higher from the side of thefirst electrode portion to the side of the second electrode portion towhich sunlight enters, and the aforementioned another cell bodypreferably has a pn-junction layer that has lower band gap than that ofthe cell body.

More specifically, the cell body preferably includes a pn-junction layerof a III-V group compound having a first band gap and a pn-junctionlayer of a III-III-V group compound having a second band gap higher thanthe first band gap, and another cell body preferably includes apn-junction layer of a III-III-V group compound having a third band gaplower than the first band gap.

Preferably, the pn-junction layer of the aforementioned another cellbody includes a pn-junction of a I-III-VI group compound.

Alternatively, the cell body preferably includes a pn-junction layer ofa III-V group compound having a first band gap arranged on the sideopposite to the sunlight entering side and a pn-junction layer of aIII-III-V group compound having a second band gap higher than the firstband gap arranged on the sunlight entering side, and the aforementionedanother cell body is preferably formed of a silicon solar battery cell.

According to another aspect, the present invention provides a method ofmanufacturing a compound solar battery including the following steps. Ona surface of a semiconductor substrate, a layer to be a first cellhaving a first band gap is formed by epitaxial growth. On the layer tobe the first cell, a layer to be a second cell having a second band gaplower than the first band gap is formed. On the layer to be the secondcell, a first electrode portion having a prescribed thickness to supportthe layers to be the first and second cells is directly formed. Thelayer to be the first cell is separated from the semiconductorsubstrate. On that surface of the layer to be the first cell which isexposed by separation from the semiconductor substrate, a secondelectrode portion is formed.

According to this manufacturing method, the layer to be the first cell,which will be positioned to the sunlight entering side in the finishedstate is formed first on the semiconductor substrate, and the layer tobe the second cell, which will be positioned to the side opposite to thesunlight entering side, is formed later. Therefore, even when a materialhaving relatively high band gap as the second band gap is used for thelayer to be the second cell, the quality of the layer to be the secondcell does not affect the layer to be the first cell. Further, as thefirst electrode portion is formed directly on the layer to be the secondcell, the component of sunlight that is not absorbed by the layers to bethe first and second cells is reflected by the first electrode portion.This improves the effect of light confinement. As a result, conversionefficiency of the compound solar battery can be improved.

Preferably, the manufacturing method further includes, between the stepof forming the layer to be the first cell and the step of forming thelayer to be the second cell, a step of forming a layer to be a thirdcell having a third band gap lower than the first band gap and higherthan the second band gap.

Thus, components of the sunlight having prescribed wavelengths areabsorbed by the respective layers to be the cells in accordance withrespective band gaps, and therefore conversion efficiency can further beimproved.

Preferably, the manufacturing method specifically includes, in order toseparate the semiconductor substrate, a step of forming a prescribedintermediate layer by epitaxial growth between the layer to be the firstcell and the semiconductor substrate, and the step of separating thelayer to be the first cell from the semiconductor substrate includes thestep of removing the semiconductor substrate by etching and furtherremoving the intermediate layer.

Alternatively, the manufacturing method preferably includes the step offorming a prescribed intermediate layer by epitaxial growth between thelayer to be the first cell and the semiconductor substrate, and the stepof separating the layer to be the first cell from the semiconductorsubstrate includes the step of removing the intermediate layer byetching so as to detach the semiconductor substrate.

By this step, it becomes possible to reuse the semiconductor substrate.

Preferably, in the step of forming the first electrode portion, thefirst electrode portion is formed of a transparent conductive film, andthe manufacturing method further includes, after the step of forming thefirst electrode and before the step of separating the first layer fromthe semiconductor substrate, the step of adhering, to the firstelectrode portion, the layer to be the third cell having the third bandgap lower than the second band gap.

In this case also, components of the sunlight having prescribedwavelengths are absorbed by the respective layers to be the cells inaccordance with respective band gaps, and therefore conversionefficiency can be improved.

In order to separate the semiconductor substrate, an intermediate layermay be formed and the semiconductor substrate may be removed by etching,or the intermediate layer may be removed by etching and thesemiconductor substrate may be detached, as described above.Particularly, when the semiconductor substrate is detached, it becomespossible to reuse the semiconductor substrate.

Preferably, the step of adhering the layer to be the third cell to thefirst electrode portion specifically includes the step of forming atransparent conductive film on a surface of the third cell and adheringthe third cell to the first electrode portion by a transparentconductive adhesive.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section representing a step of manufacturing acompound solar battery in accordance with a first embodiment of thepresent invention.

FIG. 2 is a cross section representing a step following the step shownin FIG. 1 in accordance with the first embodiment.

FIG. 3 is a cross section representing a step following the step shownin FIG. 2 in accordance with the first embodiment.

FIG. 4 is a cross section representing a step following the step shownin FIG. 3 in accordance with the first embodiment.

FIG. 5 is a cross section representing a step following the step shownin FIG. 4 in accordance with the first embodiment.

FIG. 6 is a cross section representing a step following the step shownin FIG. 5 in accordance with the first embodiment.

FIG. 7 is a perspective view showing an appearance of the finishedcompound solar battery in accordance with the first embodiment.

FIG. 8 is a partial sectional view illustrating the effect of thecompound solar battery in accordance with the first embodiment.

FIG. 9 is a partial sectional view showing a function of a comparativecompound solar battery, to demonstrate the effect of the compound solarbattery in accordance with the first embodiment.

FIG. 10 represents current-voltage characteristic of the compound solarbattery in accordance with the first embodiment, obtained by a solarsimulator.

FIG. 11 is a cross section representing a compound solar battery inaccordance with a second embodiment of the present invention.

FIG. 12 is a cross section of a comparative compound solar battery, todemonstrate the effect of the compound solar battery in accordance withthe second embodiment.

FIG. 13 is a cross section representing a step of manufacturing acompound solar battery in accordance with a third embodiment of thepresent invention.

FIG. 14 is a cross section representing a step following the step shownin FIG. 13 in accordance with the third embodiment.

FIG. 15 is a cross section representing a step following the step shownin FIG. 14 in accordance with the third embodiment.

FIG. 16 is a cross section representing a step following the step shownin FIG. 15 in accordance with the third embodiment.

FIG. 17 is a cross section representing a step following the step shownin FIG. 16 in accordance with the third embodiment.

FIG. 18 is a cross section representing a step following the step shownin FIG. 17 in accordance with the third embodiment.

FIG. 19 represents current-voltage characteristic of the compound solarbattery in accordance with the first embodiment, obtained by a solarsimulator.

FIG. 20 is a cross section representing a step of manufacturing acompound solar battery in accordance with a fourth embodiment of thepresent invention.

FIG. 21 is a cross section representing a step following the step shownin FIG. 20 in accordance with the fourth embodiment.

FIG. 22 is a cross section representing a step following the step shownin FIG. 21 in accordance with the fourth embodiment.

FIG. 23 is a cross section representing a step following the step shownin FIG. 22 in accordance with the fourth embodiment.

FIG. 24 is a cross section representing a step following the step shownin FIG. 23 in accordance with the fourth embodiment.

FIG. 25 is a cross section representing a step following the step shownin FIG. 24 in accordance with the fourth embodiment.

FIG. 26 is a cross section representing a step of manufacturing acompound solar battery in accordance with a fifth embodiment of thepresent invention.

FIG. 27 is a cross section representing a step following the step shownin FIG. 26 in accordance with the fifth embodiment.

FIG. 28 is a cross section representing a step following the step shownin FIG. 27 in accordance with the fifth embodiment.

FIG. 29 is a cross section representing a step following the step shownin FIG. 28 in accordance with the fifth embodiment.

FIG. 30 is a perspective view showing an appearance of the finishedcompound solar battery in accordance with the fifth embodiment.

FIG. 31 is a cross section representing a step of manufacturing acompound solar battery in accordance with a sixth embodiment of thepresent invention.

FIG. 32 is a cross section representing a step following the step shownin FIG. 31 in accordance with the sixth embodiment.

FIG. 33 is a perspective view showing an appearance of the finishedcompound solar battery in accordance with the sixth embodiment.

FIG. 34 is a cross section representing a conventional solar battery.

FIG. 35 is a cross section representing another conventional solarbattery.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

A compound solar battery in accordance with the first embodiment of thepresent invention will be described. Here, a 2-junction type compoundsolar battery having a bottom cell and a top cell will be described asan example of the cell body of the compound solar battery.

First, the manufacturing method will be described. As a substrate, aGaAs substrate (1×10¹⁸ cm⁻³, Si doped, 50 mm in diameter) is prepared.The GaAs substrate is put in a vertical MOCVD (Metal Organic ChemicalVapor Deposition) apparatus.

Thereafter, as shown in FIG. 1, an n-type InGaP layer 3 having thethickness of about 0.5 μm is formed by epitaxial growth on a surface ofGaAs substrate 1. InGaP layer 3 will be an intermediate layer between acell body that will be formed on InGaP layer 3 and GaAs substrate 1.

Thereafter, on InGaP layer 3, single crystal layers to be the top cell Tare formed by epitaxial growth. Specifically, an n-type GaAs layer T1,an n-type AlInP layer T2, an n-type InGaP layer T3, a p-type InGaP layerT4 and a p-type AlInP layer T5 are formed successively.

Thereafter, on AlInP layer T5, a p-type AlGaAs layer 5 and an n-typeInGaP layer 7 are successively formed by epitaxial growth, as a tunneljunction.

Thereafter, on n-type InGaP layer 7, various single crystal layers to bethe bottom cell B are formed by epitaxial growth. Specifically, ann-type AlInP layer B1, an n-type GaAs layer B2, a p-type GaAs layer B3,a p-type InGaP layer B4 and a p-type GaAs layer B5 are formedsuccessively.

As to the condition of epitaxial growth, temperature is set to about700° C. TMG (trimethyi gallium) and AsH₃ (arsine) are used as materialsfor growing the GaAs layer.

TMI (trimethyl indium), TMG and PH₃ (phosphine) are used as materialsfor growing the InGaP layer. TMA (trimethyl aluminum), TMI and PH₃ areused as materials for growing the AlInP layer.

As an impurity for forming n-type GaAs layer, InGaP layer and AlInPlayer, SiH₄ (monosilane) is used. As an impurity for forming p-type GaAslayer, InGaP layer and AlInP layer, DEZn (diethyl zinc) is used.

Further, TMI, TMG and AsH₃ are used as materials for growing AlGaAslayer, and as an impurity for forming p-type AlGaAs layer, CBr₄ (carbontetra-bromide) is used.

In this manner, the cell body C of a compound solar battery having a topcell T and a bottom cell B is formed.

Thereafter, on a surface of cell body C (p-type GaAs layer of the bottomcell), an Au—Zn film (not shown) is vapor-deposited. Thereafter, in anitrogen atmosphere, heat treatment of about one minute is performed ata temperature of about 400° C.

Thereafter, a resist (not shown) is applied and thermally cured on aback surface of GaAs substrate 1. Then, by electrolytic plating, an Auplated layer having the thickness of about 30 μm is formed on the Au—Znfilm.

In this manner, a back surface electrode 9 of Au plated film is formedon the cell body C. Thereafter, the resist formed on the back surface ofGaAs substrate 1 is removed.

Thereafter, as a protection at the time of etching, wax 11, for example,is applied to back surface electrode 9, and a glass plate 13 and backsurface electrode 9 are temporarily adhered together for easierhandling. Thereafter, GaAs substrate 1 supported by glass substrate 13is dipped in an alkali solution such as ammonia water, and GaAssubstrate 1 is removed.

Here, GaAs substrate 1 having the thickness of about 350 μm is fullyetched and removed by keeping dipped in an alkali solution for about 300minutes. Etching is stopped when InGaP layer 3 as the intermediate layeris exposed.

At this time, GaAs substrate 1 supported by glass substrate 13 may bedipped in an acid solution such as HCl to etch InGaP layer 3 as theintermediate layer, so as to remove GaAs substrate 1.

Thereafter, by etching with an acid solution, InGaP layer 3 as theexposed intermediate layer is removed, and n-type GaAs layer T1 of topcell T is exposed. In this manner, the surface of cell body C(specifically, top cell T) comes to be exposed as shown in FIG. 3.

Thereafter, by photolithography, a prescribed resist pattern (not shown)for forming a surface electrode is formed on the exposed surface of cellbody C (top cell).

Thereafter, cell body C with the resist pattern formed thereon isintroduced to a vacuum vapor deposition apparatus (not shown) togetherwith glass substrate 13. By resistance heating method, an Au film(containing 12% by weight of Ge) having the thickness of about 100 nm(not shown) is formed to cover the resist pattern.

Thereafter, by EB (Electron Beam) vapor deposition method, an Ni layerhaving the thickness of about 20 nm and an Au layer having the thicknessof about 5000 nm (both not shown) are formed continuously.

Thereafter, by lift-off method, the resist pattern and the Au film andthe like formed on the resist pattern are removed. In this manner,surface electrode 15 is formed as shown in FIG. 4.

Thereafter, using surface electrode 15 as a mask, etching with alkalisolution is performed to remove exposed GaAs layer, and the AlInP layeris exposed (see FIG. 6).

Thereafter, a prescribed resist pattern (not shown) for mesa etching isformed to cover surface electrode 15. Using the resist pattern as amask, etching with an alkali solution and an acid solution is performed,so that the Au plated film serving as back surface electrode 9 isexposed.

Next, by EB vapor deposition method, a TiO₂ film having the thickness ofabout 55 nm and an MgF₂ film having the thickness of about 100 nm (bothnot shown) are formed successively as anti-reflection films, on thesunlight entering side (surface). Thereafter, by removing wax 11 usingtoluene, for example, glass substrate 13 is separated from back surfaceelectrode 9, as shown in FIG. 5.

Thereafter, by cutting the Au plated film along the exposed line-shapedAu plated film, twelve compound solar batteries having the size of 10mm×10 mm, by way of example, are fabricated.

FIG. 6 shows a cross-sectional structure of the compound solar batterymanufactured in this manner. As shown in FIG. 6, as compared with thestructure of the conventional solar battery having the bottom cellformed on a prescribed substrate for epitaxial growth (see, for example,FIG. 34 or 35), the present compound solar battery has the back surfaceelectrode 9 formed directly on the bottom cell B of cell body C.Further, surface electrode 15 is formed on the surface of top cell T ofcell body C.

Cell body C includes bottom cell B having a pn-junction of GaAs (III-Vgroup compound) and top cell T having a pn-junction of InGaP (III-III-Vgroup compound).

Thickness L1 of cell body C is about 4 μm, and thickness L2 of backsurface electrode 9 is about 30 μm. Namely, cell body C and back surfaceelectrode 9 are thin enough to have flexibility, and hence, compoundsolar battery 10 can freely be deflected.

In the compound solar battery described above, on GaAs substrate1 forepitaxial growth, layers to be the top cell T are successively formed byepitaxial growth, and on the top cell T, layers to be the bottom cell Bare formed.

Thereafter, GaAs substrate 1 is separated from cell body C, back surfaceelectrode 9 is directly formed on bottom cell B, and cell body C comesto be supported by back surface electrode 9. As back surface electrode 9is directly formed on the surface of bottom cell B, the effect of lightconfinement when the sunlight enters can be improved.

Specifically, as shown in FIG. 8, in the compound solar batterydescribed above, components of the sunlight that have not been absorbedby cell body C while passing through cell body C are eventuallyreflected by back surface electrode 9. Therefore, the effect of lightconfinement of cell body C is improved, and the components of sunlightthat are reflected by back surface electrode 9 contribute to powergeneration. As a result, conversion efficiency of the solar battery cellcan be improved.

In contrast, in the conventional compound solar battery, on the bottomcell side of cell body CC, substrate 101 for epitaxial growth ispositioned, as shown in FIG. 9. Therefore, components of sunlight thathave not been absorbed by cell body CC while passing through cell bodyCC are absorbed by substrate 101, and therefore, these components do notcontribute to power generation.

The solar battery cell described above was evaluated using a solarsimulator. The result will be described. The solar simulator refers to aradiation light source used for indoor testing of characteristics andreliability of a solar battery, and required radiation illumination,uniformity and spectrum matching are satisfied in accordance with theobject of testing.

First, as the radiation light source, a reference sunlight having airmass (AM) of 1.5 G was used, and current-voltage characteristic at thetime of irradiation was measured. Based on the current-voltagecharacteristic, short circuit current, open circuit voltage, fill factorand conversion efficiency are calculated.

Here, air mass refers to a ratio of the path length of sunlight directlyincident on the earth with respect to path length of sunlight verticallyentering the atmosphere in the standard condition (standard atmosphericpressure of 1013 hPa).

The short circuit current refers to the current flowing between twooutput terminals when the output terminals of the solar battery cell(module) are short-circuited. The open circuit voltage refers to thevoltage between the two output terminals when the output terminals ofthe solar battery cell (module) are opened.

The fill factor refers to a value obtained by dividing maximum output bya product of open circuit voltage and short circuit current. Theconversion efficiency refers to a value (%) obtained by dividing themaximum output by a product of the area of solar battery cell (module)and radiation luminance.

FIG. 10 shows the measured current-voltage characteristic (I-V curve).Here, the short circuit current was 10.1 mA, open circuit voltage was2.39 V, fill factor was 0.85, and conversion efficiency was 20.5%.

From the foregoing, it was found that compared with the conventional2-junction type compound solar battery having two pn-junctions of InGaAsand GaAs formed on a GaAs substrate, the present compound solar batterydescribed above attained comparative or better results.

Second Embodiment

Here, in order to confirm the effect of light confinement attained bythe back surface electrode, a compound solar battery having a cell bodystructure different from the one described above was evaluated as anexample. This will be described in the following.

First, as shown in FIG. 11, in the compound solar battery in accordancewith the present embodiment, cell body C is directly formed on thesurface of back surface electrode 9. In cell body C, a p-type InGaPlayer 21 is formed on back surface electrode 9. On InGaP layer 21, ap-type GaAs layer 22 is formed.

On GaAs layer 22, an n-type GaAs layer 23 is formed. On GaAs layer 23,an n-type InGaP layer 24 is formed. At a prescribed position on InGaPlayer 24, a surface electrode 15 is formed, with a contact of n-typeGaAs layer interposed.

The compound solar battery is formed through the method similar to thatof the compound solar battery described above. Specifically, first,layers from n-type GaAs layer 25 to p-type InGaP layer 21 aresuccessively formed on a prescribed substrate (not shown). Thereafter,back surface electrode 9 is formed on the side of the bottom cell, andthe substrate is separated.

In contrast, in the comparative compound solar battery, on a surface ofp-type GaAs substrate 101, a p-type InGaP layer 121 is formed. On InGaPlayer 121, a p-type GaAs layer 122 is formed.

On GaAs layer 122, an n-type GaAs layer 123 is formed. On GaAs layer123, an n-type InGaP layer 124 is formed. At a prescribed position onInGaP layer 124, a surface electrode 115 is formed, with a contact ofn-type GaAs layer 125 interposed.

The comparative compound solar battery is formed by successively growingvarious layers through epitaxial growth, on a p-type GaAs substrate 101.

The compound solar battery described above and the comparative solarbattery are evaluated by using the solar simulator described above. Inthe compound solar battery in accordance with the present embodiment,the short circuit current was 19 mA, open circuit voltage was 1.03 V,fill factor was 0.84 and conversion efficiency was 16.4%.

In contrast, in the comparative compound solar battery, the shortcircuit current was 15 mA, open circuit voltage was 1.03 V, fill factorwas 0.84 and conversion efficiency was 13.0%.

As can be seen from above, in the compound solar battery in accordancewith the present embodiment, the conversion efficiency is particularlyimproved as compared with the comparative compound solar battery, andthus, it was found that the effect of light confinement by back surfaceelectrode 9 could be improved.

Third Embodiment

A compound solar battery in accordance with a third embodiment will bedescribed. Here, by way of example, a 3-junction type compound solarbattery having a bottom cell, middle cell and top cell as the cell bodyof the compound solar battery will be described.

First, manufacturing method will be described. As a substrate, a GaAssubstrate (1×10¹⁸ cm⁻³, Si doped, 50 mm in diameter) is prepared. TheGaAs substrate is put in a vertical MOCVD apparatus.

Then, as shown in FIG. 13, on GaAs substrate 1, an n-type AlAs layer 4to be the intermediate layer, having the thickness of about 0.5 μm isformed by epitaxial growth.

On AlAs layer 4, layers to be the top cell T are formed by epitaxialgrowth. Specifically, an n-type GaAs layer T1, an n-type AlInP layer T2,an n-type InGaP layer T3, a p-type InGaP layer T4 and a p-type AlInPlayer T5 are formed successively.

Thereafter, as a tunnel junction, on AlInP layer 5, a p-type AlGaAslayer 5 and n-type InGaP layer 7 are successively formed.

Thereafter, on n-type InGaP layer 7, layers to be the middle cell M areformed by epitaxial growth. Specifically, an n-type AlInP layer M1, ann-type GaAs layer M2, a p-type GaAs layer M3 and p-type InGaP layer M4are successively formed.

Thereafter, on p-type InGaP layer M4, as a tunnel junction, a p-typeGaAs layer 6 and an n-type GaAs layer 8 are successively formed byepitaxial growth.

Thereafter, on n-type GaAs layer 8, layers to be the bottom cell B areformed by epitaxial growth. Specifically, an n-type InP layer B6, ann-type InGaAs layer B7, a p-type InGaAs layer B8, a p-type InP layer B9and a p-type GaAs layer B10 are successively formed.

As to the condition of epitaxial growth, temperature is set to about700° C. TMG (trimethyl gallium) and AsH₃ (arsine) are used as materialsfor growing the GaAs layer.

TMI (trimethyl indium), TMG and PH₃ (phosphine) are used as materialsfor growing the InGaP layer. TMA (trimethyl aluminum), TMI and PH₃ areused as materials for growing the AlInP layer.

As an impurity for forming n-type GaAs layer, InGaP layer and AlInPlayer, SiH₄ (monosilane) is used. As an impurity for forming p-type GaAslayer, InGaP layer and AlInP layer, DEZn (diethyl zinc) is used.

Further, TMI, TMG and AsH₃ are used as materials for growing AlGaAslayer, and as an impurity for forming p-type AlGaAs layer, CBr₄ (carbontetra-bromide) is used.

Composition ratio of In in InGaAs layer is 0.25, and on the InGaAslayer, a morphology of cross-hatch pattern indicating presence of misfitdislocation was observed.

In this manner, the cell body C of a 3-junction type compound solarbattery including top cell T, middle cell M and bottom cell B is formed.

On the surface of cell body C (p-type GaAs layer of the bottom cell), aresist pattern (not shown) for forming the back surface electrode isformed. An Au—Zn film (not shown) is vapor-deposited to cover the resistpattern.

Thereafter, by the lift-off method, the resist pattern and the Au—Znfilm positioned on the resist pattern are removed. Thereafter, in anitrogen atmosphere, heat treatment of about 1 minute is performed at atemperature of about 400° C.

Then, except for the regions where the Au—Zn film is formed, aprescribed resist pattern (not shown) is formed. Further, on a surfaceof GaAs substrate 1 on the side where cell body C is not formed, aresist (not shown) is applied.

Thereafter, an Au plated film (not shown) having the thickness of about30 μm is formed on the Au—Zn film, by electrolytic plating. Thereafter,by the lift-off method, the resist pattern and the Au plated filmpositioned on the resist pattern are removed. Consequently, back surfaceelectrode 9 of Au plated film is formed on the cell body, as shown inFIG. 14.

Thereafter, a prescribed resist pattern 17 is formed to cover backsurface electrode 9 in a region where back surface electrode 9 is formedand to expose the surface of cell body C in a region where back surfaceelectrode 9 is not formed, as shown in FIG. 15.

Using the resist pattern 17 as a mask, etching with an alkali solutionand an acid solution is performed, so that the portion of the exposedcell body C is removed and AlAs layer 3 as the intermediate layer isexposed. Thereafter, resist pattern 17 is removed.

Thereafter, a mesh-shaped resin plate 19 having chemical resistance isadhered to the side of back surface electrode 9, with wax 11 interposed(see FIG. 16). With resin plate 19 adhered on back surface electrode 9,the cell body C and back surface electrode 9 are dipped in ahydrofluoric acid solution.

Dipped in the hydrofluoric acid solution, AlAs layer 4 is removed, andtherefore, the cell body C is separated from GaAs substrate 1. In thismanner, GaAs substrate 1 is separated and the n-type GaAs layer of thetop cell T of cell body C is exposed.

Thereafter, on the exposed surface of GaAs layer, a prescribed resistpattern for forming a surface electrode (not shown) is formed.Thereafter, cell body C with the resist pattern formed thereon isintroduced to a vacuum vapor deposition apparatus (not shown) togetherwith resin plate 19.

By resistance heating method, an Au film (containing 12% by weight ofGe) having the thickness of about 100 nm is formed to cover the resistpattern. Thereafter, by EB (Electron Beam) vapor deposition method, anNi layer having the thickness of about 20 nm and an Au layer having thethickness of about 5000 nm (both not shown) are formed continuously.

Thereafter, by lift-off method, the resist pattern and the Au film andthe like formed on the resist pattern are removed. In this manner,surface electrode 15 is formed as shown in FIG. 17.

Thereafter, using surface electrode 15 as a mask, etching with alkalisolution is performed to remove exposed GaAs layer, and the AlInP layeris exposed (see FIG. 18).

Next, by EB vapor deposition method, a TiO₂ film having the thickness ofabout 55 nm and an MgF₂ film having the thickness of about 100 nm (bothnot shown) are formed successively as anti-reflection films, on thesunlight entering side (surface). Thereafter, by removing wax 11 usingtoluene, for example, resin plate 19 is separated from back surfaceelectrode 9, as shown in FIG. 18.

Thereafter, by cutting the Au plated film along the exposed line-shapedAu plated film, twelve compound solar batteries having the size of 10mm×10 mm, by way of example, are fabricated.

FIG. 18 shows a cross-sectional structure of the compound solar batterymanufactured in this manner. As shown in FIG. 18, as compared with thestructure of the conventional solar battery having the bottom cellformed on a prescribed substrate for epitaxial growth (see, for example,FIG. 34 or 35), the present compound solar battery has the back surfaceelectrode 9 formed directly on the bottom cell B of cell body C.

Further, surface electrode 15 is formed on the surface of top cell T ofcell body C. Middle cell M is formed between top cell T and bottom cellB.

The cell body C includes a bottom cell having a pn-junction of InGaAs(III-III-V group compound), a middle cell M having a pn-junction of GaAs(III-V group compound) and a top cell T having a pn-junction of InGaP(III-III-V group compound).

Thickness L1 of cell body C is about 6 μm, and thickness L2 of backsurface electrode 9 is about 30 μm. Namely, cell body C and back surfaceelectrode 9 are thin enough to have flexibility, and hence, compoundsolar battery 10 can freely be deflected, as in the compound solarbattery described above.

In the solar battery cell described above, on the GaAs substrate 1 forepitaxial growth, layers to be the top cell T having the band gap ofabout 1.7 to about 2.1 eV are successively formed by epitaxial growth.

Then, on the top cell T, layers to be the middle cell M having the bandgap of about 1.3 to about 1.6 eV are successively formed. Further, onthe middle cell M, layers to be the bottom cell B having the band gap ofabout 0.9 to 1.1 eV are successively formed.

In this manner, in the compound solar battery described above, layers tobe the top cell T are formed first and the layers to be the bottom cellB are formed last.

Therefore, even when a material having larger band gap (about 0.9 toabout 1.1 eV) than a conventional material (˜0.7 eV) is used for thebottom cell B, the quality of the bottom cell B does not have anyinfluence on the middle cell M and the top cell T, and the conversionefficiency of the compound solar battery can be improved. This will bedescribed in detail in the following.

In the conventional method of manufacturing a compound solar battery, onthe Ge substrate (or GaAs substrate) for epitaxial growth, layers to bethe bottom cell are formed first, and layers to be the top cell areformed later.

Here, when InGaAs having a relatively high band gap (about 0.9 to about1.1 eV) is applied as the material of the bottom cell, misfitdislocation results in the InGaAs layer, as the lattice constant of theGe substrate (GaAs substrate) is different from that of InGaAs.

When InGaAsN is applied as the bottom cell, defects related to N atomsare generated in InGaAsN.

Such defects or misfit dislocation generated in the bottom cell affectsthe GaAs layer to be the middle cell epitaxially grown on the bottomcell as well as the InGaP layer to be the top cell.

Consequently, quality of the middle cell and the top cell is degraded,making it difficult to improve conversion efficiency of the compoundsolar battery.

In contrast, in the compound solar battery described above, layers to bethe top cell and layers to be the middle cell are successively formed onthe surface of GaAs substrate 1, and layers to be the bottom cell areformed last.

Here, lattice constant of InGaAs to be the bottom cell is different fromthat of GaAs to be the middle cell. Therefore, the quality of the bottomcell formed on the middle cell is comparative to that of theconventional compound solar battery.

On the other hand, lattice constant of InGaP to be the top cell andlattice constant of GaAs to be the middle cell are the same as latticeconstant of GaAs substrate for epitaxial growth. Therefore, dislocationor defect is not generated in the InGaP and GaAs layers epitaxiallygrown successively on the GaAs substrate 1.

Specifically, in the compound solar battery described above, even whenthe quality of bottom cell B is comparative to the quality of bottomcell of the conventional compound solar battery, degradation in qualityof the bottom cell does not have any influence on the middle cell M ortop cell T, as the top cell T and middle cell M are formed earlier.

As a result, even when a material having relatively high band gap suchas InGaAs is used for the bottom cell B, quality of the middle cell andthe top cell is not degraded, and conversion efficiency of the compoundsolar battery can be improved.

Evaluation of the above described compound solar battery made by thesolar simulator will be described in the following. FIG. 19 shows themeasured current-voltage characteristic (I-V curve). Here, the shortcircuit current was 10.2 mA, open circuit voltage was 2.49 V, fillfactor was 0.85 and conversion efficiency was 21.6%.

From these results, it was found that the compound solar batterydescribed above had higher open circuit voltage and higher conversionefficiency, as compared with the conventional 2-junction type compoundsolar battery having two pn-junctions of InGaAs and GaAs formed on aGaAs substrate.

Fourth Embodiment

A compound solar battery in accordance with a fourth embodiment of thepresent invention will be described. Here, another example of a3-junction type compound solar battery having bottom cell, middle celland top cell as the cell body will be described.

First, manufacturing method will be described. In the similar manner asdescribed in the first embodiment, layers to be the top cell and layersto be the middle cell are successively formed on GaAs substrate 1, asshown in FIG. 20.

On InGaP layer M4 of the middle cell M, a p-type GaAs layer 6 and ann-type GaAs layer 8 are successively formed as a tunnel junction.Thereafter, on GaAs layer 8, layers to be the bottom cell B are formed.

Specifically, an ITO (Indium Tin Oxide) film 10, a CdS film B11 andCuInSe₂ film B12 are successively formed. ITO film 12 is formed, by wayof example, by sputtering. CdS film B11 is formed, by way of example, byvapor deposition. CuInSe₂ film is formed, by way of example, by vapordeposition.

In this manner, the cell body C of the 3-junction type compound solarbattery including top cell T, middle cell M and bottom cell B is formed.

Thereafter, on the surface of cell body C (p-type CuInSe₂ film of thebottom cell), a prescribed resist pattern (not shown) for forming theback surface electrode is formed. An Mo film (not shown) isvapor-deposited to cover the resist pattern.

Thereafter, by lift-off method, the resist pattern and the Mo filmpositioned on the resist pattern are removed. Thereafter, in a nitrogenatmosphere, heat treatment of about one minute is performed at atemperature of about 400° C.

Thereafter, a prescribed resist pattern (not shown) is formed except fora region on which the Mo film is formed. Further, a resist (not shown)is applied to that side of the GaAs substrate 1 on which cell body C isnot formed.

Thereafter, an Au plated layer (not shown) having the thickness of about30 μm is formed on the Mo film by electrolytic plating. Thereafter, bylift-off method, the resist pattern and the Au plated film positioned onthe resist pattern are removed. In this manner, a back surface electrode9 of Au plating is formed on the cell body, as shown in FIG. 21.

Thereafter, a prescribed resist pattern 17 is formed to cover backsurface electrode 9 in a region where back surface electrode 9 is formedand to expose the surface of cell body C in a region where back surfaceelectrode 9 is not formed, as shown in FIG. 22.

Using the resist pattern 17 as a mask, prescribed etching is performed,so that the portion of the exposed cell body C is removed and AlAs layer3 as the intermediate layer is exposed. Thereafter, resist pattern 17 isremoved.

Thereafter, as shown in FIG. 23, a mesh-shaped resin plate 19 havingchemical resistance is adhered to the side of back surface electrode 9,with wax 11 interposed. With resin plate 19 adhered on back surfaceelectrode 9, the cell body C and back surface electrode 9 are dipped ina hydrofluoric acid solution.

Dipped in the hydrofluoric acid solution, AlAs layer 3 is removed, andtherefore, the cell body C is separated from GaAs substrate 1. In thismanner, GaAs substrate 1 is separated and the n-type GaAs layer of thetop cell of cell body C is exposed.

Thereafter, on the exposed surface of GaAs layer, a prescribed resistpattern for forming a surface electrode (not shown) is formed.Thereafter, cell body C with the resist pattern formed is introduced.Thereafter, cell body C with the resist pattern formed thereon isintroduced to a vacuum vapor deposition apparatus (not shown) togetherwith resin plate 19.

By resistance heating method, an Au film (containing 12% by weight ofGe) having the thickness of about 100 nm is formed to cover the resistpattern. Thereafter, by EB (Electron Beam) vapor deposition method, anNi layer having the thickness of about 20 nm and an Au layer having thethickness of about 5000 nm (both not shown) are formed continuously.

Thereafter, by lift-off method, the resist pattern and the Au film andthe like formed on the resist pattern are removed. In this manner,surface electrode 15 is formed as shown in FIG. 24.

Thereafter, using surface electrode 15 as a mask, etching with alkalisolution is performed to remove exposed GaAs layer, and the AlInP layeris exposed (see FIG. 25).

Next, by EB vapor deposition method, a TiO₂ film having the thickness ofabout 55 nm and an MgF₂ film having the thickness of about 100 nm (bothnot shown) are formed successively as anti-reflection films, on thesunlight entering side (surface). Thereafter, by removing wax 11 usingtoluene, for example, resin plate 19 is separated from back surfaceelectrode 19, as shown in FIG. 25.

Thereafter, by cutting the Au plated film along the exposed line-shapedAu plated film, twelve compound solar batteries having the size of 10mm×10 mm, by way of example, are fabricated.

FIG. 25 shows a cross-sectional structure of the compound solar batterymanufactured in this manner. As shown in FIG. 25, as compared with thestructure of the conventional solar battery having the bottom cellformed on a prescribed substrate for epitaxial growth (see, for example,FIG. 34 or 35), the present compound solar battery has the back surfaceelectrode 9 formed directly on the bottom cell B of cell body C.

Further, surface electrode 15 is formed on the surface of top cell T ofcell body C. Middle cell M is formed between top cell T and bottom cellB. Thus, a 3-junction type compound solar battery is provided, whichincludes as the cell body, the bottom cell B, middle cell M and top cellT.

Particularly, different from the top cell T and the middle cell M thatare formed by epitaxial growth, the bottom cell B has CdS film B11 andCuInSe₂ film B12 that are formed by vapor deposition.

Therefore, the cell body C includes a bottom cell having a pn-junctionof CuInSe₂ (I-III-VI group compound) and CdS (II-VI group compound), amiddle cell M having a pn-junction of GaAs (III-V group compound) and atop cell T having a pn-junction of InGaP (III-III-V group compound).

In the solar battery cell described above, on the GaAs substrate 1 forepitaxial growth, layers to be the top cell T having the band gap ofabout 1.7 to about 2.1 eV are successively formed by epitaxial growth.

Then, on the top cell T, layers to be the middle cell M having the bandgap of about 1.3 to about 1.6 eV are successively formed. Further, onthe middle cell M, layers to be the bottom cell B having the band gap ofabout 0.9 to 1.1 eV are successively formed by sputtering and vapordeposition, different from epitaxial growth.

In this manner, in the compound solar battery described above, layers tobe the top cell are formed first and the layers to be the bottom cellare formed last. Therefore, even when a material having larger band gap(about 0.9 to about 1.1 eV) than a conventional material (˜0.7 eV) isused for the bottom cell B, the quality of the bottom cell B does nothave any influence on the middle cell M and the top cell T, and theconversion efficiency of the compound solar battery can be improved.

Further, as the quality of bottom cell B does not have any influence onthe top cell T and middle cell M, layers to be the bottom cell B can beformed by a method other than epitaxial growth.

Therefore, as the material of the layers to be the bottom cell having arelatively high band gap (0.9 eV˜1.1 eV), a material other than a singlecrystal material, such as polycrystalline CuInSe₂ film B12 can beapplied, and thus, material of the layers to be the bottom cell and themethod of manufacturing the same can be selected from wider variety.

Fifth Embodiment

A compound solar battery in accordance with a fifth embodiment of thepresent invention will be described. Here, another example of 2-junctiontype compound solar battery will be described. First, manufacturingmethod will be described. In the similar manner as described in thefirst embodiment, on GaAs substrate 1, layers T1 to T5 to be the topcell T and layers B13, B2, B3, B14 to be the bottom cell B aresuccessively formed, with an intermediate layer (n-type AlAs layer 21)interposed, to form a cell body C of the 2-junction type compound solarbattery including top cell T and bottom cell B, as shown in FIG. 26.

Thereafter, in the similar manner as described in the first embodiment,back surface electrode 9 of Au plated film is formed on the cell body C,as shown in FIG. 27. Thereafter, a film material 22 having erosionresistance, heat resistance and weather resistance such as a kapton(registered trademark) tape is adhered to back surface electrode 9, asshown in FIG. 28. Wax 11 is applied to film material 22 for protection.

Then, GaAs substrate 1 on which cell body C is formed is dipped, forexample, in a mixed solution of hydrofluoric acid and water(HF:H₂O=1:10) for about 5 hours, so that the intermediate layer (N-typeAlAs layer 21) having the thickness of about 5 to about 10 nm positionedbetween the cell body C and GaAs substrate 1 is etched and the cell bodyC is separated from GaAs substrate 1 (not shown), as shown in FIG. 29.

The surface of the separated GaAs substrate 1 is not etched but kept ina mirror finished state, and therefore, the substrate can be used againas a substrate for epitaxial growth.

Thereafter, in the similar manner as described in the first embodiment,surface electrode 15 is formed on the surface of cell body C. In thismanner, a 2-junction type solar battery is formed as shown in FIG. 30.

In the compound solar battery described above, back surface electrode 9is formed on a prescribed film member 22 and on back surface electrode9, cell body C is directly formed. Surface electrode 15 is formed oncell body C. The compound solar battery having the film member 22 andcell body C integrated together can be directly applied as a solarbattery panel.

In the above described embodiments, a back surface electrode of Auplated film having the thickness of about 30 μm has been mainlydescribed as the back surface electrode 9. Thickness of back surfaceelectrode 9, however, is not limited thereto, provided that it is thickenough to support the cell body C.

Therefore, back surface electrode 9 may have such a thickness thatallows deflection. Alternatively, back surface electrode 9 may have sucha thickness that allows deflection dependent on the material of theelectrode 9.

In that case, the compound solar battery having the back surfaceelectrode 9 formed on cell body C can freely be deflected, and degree offreedom in shape is improved.

Sixth Embodiment

Here, a compound solar battery including a cell body having one surfaceelectrode of one polarity formed on a light entering side and atransparent conductive film to be a back surface electrode of the otherpolarity formed on the back surface side, and another cell adhered tothe transparent conductive film, will be described.

In the compound solar battery, the cell body or the aforementionedanother cell body is formed of a single crystal thin film formed byepitaxial growth. As the substrate used for the epitaxial growth iscompletely removed, the compound solar battery can be made thin andefficiency can be improved. Further, the compound solar battery is hardto break.

Between the cell body formed of the epitaxially grown single crystalthin film and another cell body holding the same, the transparentconductive film to be the back surface electrode is formed. Therefore,electric resistance from the epitaxial layer can sufficiently belowered. Further, if the holding material itself is made conductive,electric resistance between the holding material and the epitaxial layercan further be lowered.

Preferable material for forming the transparent conductive film may havetransmittance of at least 70% of light having wavelength of 850 nm orlonger and resistance of at most 1Ω·cm, and such material includes ITO,In₂O₃, SnO₂, ZnO, CdO, TiO₂, CdIn₂O₄, Cd₂SnO₄ and Zn₂SnO₄.

As already described, in order to improve power generation efficiency ofa compound solar battery, it is effective to superpose a plurality ofsolar batteries (multi-junction) formed of materials having differentabsorption wavelengths. Specifically, it is preferred in the cell bodyof the compound solar battery that a cell having a pn-junction layerformed of a material having a relatively high band gap on the sunlightentering side and a cell having a pn-junction layer formed of a materialhaving a relatively low band gap on the back surface side are formed,and that such a plurality of cells are joined by a tunnel junctionlayer.

Further, it is preferred in another cell body adhered through thetransparent conductive film, that a pn-junction layer formed of amaterial having a band gap still lower than the band gap of the cell ofthe cell body is provided.

Specifically, it is preferred that in the cell body, a cell having apn-junction layer formed of (Al)InGa(As)P single crystal material (bandgap: 1.7˜2.1 eV) is arranged on the light entering side and a cellhaving a pn-junction formed of (Al)(In)GaAs(P) single crystal material(band gap: 1.3˜1.6 eV) is arranged on the back surface side, and thatthe plurality of such cells are joined by tunnel junction. Further, itis preferred that in another cell body, a cell having a pn-junctionlayer formed of InGaAs(P) single crystal material (band gap: 0.7˜1.2 eV,more preferably, 0.9˜1.1 eV) is arranged.

As the aforementioned another cell body, a cell having a pn-junctionformed of a I-III-VI group compound of CuInGaSeS material is morepreferable. The band gap of the cell body is preferably 0.7˜1.2 eV and,more preferably, 0.9˜1.1 eV.

Alternatively, it is preferred that in the cell body, a cell having apn-junction layer formed of (Al)InGa(As)P single crystal material (bandgap: 1.8˜2.1 eV) is arranged on the light entering side and a cellhaving a pn-junction formed of (Al)(In)GaAs(P) single crystal material(band gap: 1.4˜1.6 eV) is arranged on the back surface side, and thatthe plurality of such cells are joined by tunnel junction.

As the aforementioned another cell body, an Si solar battery ispreferred. As the Si solar battery, a single crystal Si solar battery ismore preferred, as the solar battery itself serves as a substrate andattains high power generation efficiency.

In the method of manufacturing the multi-junction type compound solarbattery, when layers to be the cell are to be epitaxially grown after anintermediate layer is formed on a semiconductor substrate, layers aregrown starting from the layer positioned on the light entering side ofthe solar battery to the layer positioned on the back side, in adirection opposite to the conventional manufacturing method, so that asurface of the cell to be the bottom is exposed.

Thereafter, a transparent conductive film to be the back surfaceelectrode of the compound solar battery is formed on the surface of theexposed layer, and another cell body having a certain strength isadhered to the transparent conductive film. Thereafter, thesemiconductor substrate for epitaxial growth is removed.

The semiconductor substrate is separated at the middle layer positionedbetween the semiconductor substrate and the cell body. Thus, thesemiconductor substrate can be removed without damaging the cell bodyand another cell body. When the thickness of the compound semiconductorbecomes 10 μm or thinner, elasticity of the semiconductor improves andit becomes harder to break. Therefore, a compound solar battery that ishard to break can be manufactured through the above described manner.

Further, as the semiconductor substrate is not made thin but completelyremoved, breakage of the substrate caused by unevenness of the compoundsolar battery can be prevented. Further, as the unnecessary substrate iseliminated, the weight of the compound solar battery can be reduced, andpower generation efficiency can be improved.

When the intermediate layer left after the removal of the semiconductorsubstrate is removed, the surface of the cell body comes to be exposed.By forming a prescribed surface electrode or the like on the exposedsurface, a multi-junction type compound solar battery can be obtained.

As to the solvent used for separating the semiconductor substrate fromthe cell body, it is preferred that the semiconductor substrate isremoved quickly, dissolution is stopped at the intermediate layer andthe dissolution of the cell body is prevented, and therefore, a solvent,of which solubility of semiconductor substrate is higher than that ofthe intermediate layer is preferred. Specifically, a solvent, of whichsolubility of the semiconductor substrate is at most 5% of thesolubility of the intermediate layer is preferred, and at most 3% ismore preferred.

When GaAs, Ge or the like is used as the material of the semiconductorsubstrate and InGaP, AlInP, AlAs or the like is used as the material ofthe intermediate layer, an acid solution of concentrated hydrochloricacid is preferred.

When the aforementioned another cell body is adhered to the transparentconductive film, electric resistance at the junction portion should besufficiently lowered. Therefore, preferably, the transparent conductivefilm is formed on a surface of the said another cell, and thetransparent conductive film on the said another cell is adhered to thetransparent conductive film on the device, with a transparent conductiveadhesive interposed.

As the transparent conductive adhesive, one having transmittance of atleast 70% with respect to light of the wavelength of 850 nm or longerand having resistance of 1Ω·cm or lower should be used, and ink of ITO,In₂O₃, SnO₂, ZnO, CdO, TiO₂, CdIn₂O₄, Cd₂SnO₄ or Zn₂SnO₄ is suitable.

The method of manufacturing the compound solar battery will be morespecifically described. First, in the similar manner as described in thefirst embodiment, on a surface of GaAs substrate 1, layers C11 to C15 tobe a top cell C1, p-type AlGaAs layer 5 and n-type InGaP layer 7 to bethe tunnel junction, and layers C21 to C24 to be a bottom cell C2 areformed successively, with an intermediate layer (n-type InGaP layer 3)interposed, as shown in FIG. 31. On layer C24, a p-type GaAs layer 21and an n-type GaAs layer 22 to be the tunnel junction are formed. Inthis manner, one cell body C including top cell C1 and bottom cell C2 isformed. On the surface of n-type GaAs layer 22, a transparent conductivefilm (ITO film 1) 33 (see FIG. 33) to be an n-type ohmic electrode isformed by sputtering.

Thereafter, layers C31 to C34 to be a cell are successively formed on anInP substrate 30, and another cell body CA including cell 3 is formed,as shown in FIG. 32. On a surface of n-type InP layer C34 of cell C3, atransparent conductive film (ITO film 2) 31 (see FIG. 33) to be an ohmicelectrode is formed by sputtering.

Thereafter, on transparent conductive film 33 (ITO film 1) andtransparent conductive film 31 (ITO film 2), liquid ITO ink is applied,and the ink-applied surfaces are adhered together. In the adhered state,pre-baking at 200° C. is performed, and thereafter, ITO ink is dried andsintered at 400° C. for 60 minutes, whereby an ITO ink sintered layer 32(see FIG. 33) is formed. Transparent conductive films 31 and 33 and ITOink sintered layer 32 constitute the back surface electrode 9.

Thereafter, GaAs substrate 1 is dipped in an alkali solution to etchGaAs substrate 1, whereby GaAs substrate 1 is removed. GaAs substrate 1having the thickness of 350 μm is fully etched and removed after about300 minutes. Etching is stopped at the intermediate layer (n-type InGaPlayer 3).

Thereafter, the intermediate layer (n-type InGaP layer 3) is etched byan acid solution, so that the remaining intermediate layer is removedand n-type GaAs layer C11 to be an n-type cap layer is exposed.Thereafter, through similar steps as described in the first embodiment,a surface electrode 15 is formed on the exposed surface of n-type GaAslayer C11. Thereafter, using the surface electrode 15 as a mask, etchingwith an alkali solution is performed, whereby exposed n-type GaAs layerC11 is removed and AlInP layer C12 is exposed.

Thereafter, a prescribed resist pattern (not shown) for mesa etching isformed to cover surface electrode 15. Using the resist pattern as amask, etching with an alkali solution and an acid solution is performed,so that epitaxial layers are removed and transparent conductive film 33is exposed. By etching with an acid solution, exposed ITO film isremoved, and further by a prescribed etching, InGaAsP layers C32 and C31are removed and InP substrate 30 is exposed.

Then, on the back surface of InP substrate 30, an Au—Zn film isvapor-deposited, and by heat treatment for one minute at 400° C. in anitrogen atmosphere, an ohmic electrode 34 is formed. Further, a TiO₂film (having the thickness of 55 nm) and an MgF₂ film (having thethickness of 100 nm) may be formed continuously on the surface, as ananti-reflection film, by EB vapor deposition method.

By cleavage at portions of InP substrate 30 that are exposed by mesaetching, twelve compound solar batteries having the size of 10 mm×10 mm,for example, are fabricated.

FIG. 33 shows a cross sectional structure of the compound solar batterymanufactured in this manner. As can be seen from FIG. 33, one cell bodyC is formed on the light entering side and another cell body CA isformed on the side opposite to the light entering side, with a backsurface electrode 9 formed of a transparent conductive materialinterposed.

Using a reference sunlight having air mass (AM) of 1.5 G, the compoundsolar battery described above was evaluated by a solar simulator, andcurrent-voltage characteristic at the time of irradiation, short circuitcurrent, open circuit voltage, fill factor and conversion efficiencywere measured.

In the embodiments above, back surface electrode 9 may be formed, forexample, by printing or spraying, other than the above described platingmethod.

Assuming that the back surface electrode has such a form as describedabove, preferable thickness of back surface electrode 9 is about 2 toabout 500 μm.

In each of the compound solar batteries described above, the substratefor epitaxial growth is eventually removed, and therefore, thermalconductivity between the cell body and the heat sink is improved. As aresult, temperature increase of the cell body of the compound solarbattery can be suppressed.

Further, the removed substrate for epitaxial growth can be used again,enabling cost reduction.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

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 21. A method of manufacturing a compound solar battery,comprising the steps of: forming a layer to be a first cell having afirst band gap on a surface of a semiconductor substrate by epitaxialgrowth, forming a layer to be a tunnel junction on said layer to be thefirst cell by epitaxial growth, forming a layer to be a second cellhaving a second band gap lower than said first band gap, on said layerto be the tunnel junction, forming a first electrode portion having aprescribed thickness to support said layer to be the first cell and saidlayer to be the second cell, directly on said layer to be the secondcell, separating said layer to be the first cell from said semiconductorsubstrate, and forming a second electrode portion on a surface of saidlayer to be the first cell, exposed by separation from saidsemiconductor substrate.
 22. The method of manufacturing a compoundsolar battery according to claim 21, further comprising the steps of:between said step of forming a layer to be a tunnel junction and saidstep of forming a layer to be a second cell, forming a layer to be athird cell having a third band gap lower than said first band gap andhigher than said second band gap, on said layer to be the tunneljunction, and forming a layer to be another tunnel junction on saidlayer to be the third cell by epitaxial growth, wherein, in said step offorming a layer to be a second cell, said layer to be the second cell isformed on said layer to be another tunnel junction.
 23. The method ofmanufacturing a compound solar battery according to claim 22, wherein acompound semiconductor substrate is employed as said semiconductorsubstrate, said layer to be the first cell, said layer to be the secondcell, and said layer to be the third cell are compound semiconductors.24. The method of manufacturing a compound solar battery according toclaim 22, wherein a lattice constant of each of said layer to be thefirst cell and said layer to be the third cell takes a value identicalto the lattice constant of said semiconductor substrate.
 25. The methodof manufacturing a compound solar battery according to claim 21, whereina compound semiconductor substrate is employed as said semiconductorsubstrate, and said layer to be the first cell and said layer to be thesecond cell are compound semiconductors.
 26. The method of manufacturinga compound solar battery according to claim 21, wherein a latticeconstant of each of said layer to be the first cell and said layer to bethe second cell takes a value identical to the lattice constant of saidsemiconductor substrate.
 27. The method of manufacturing a compoundsolar battery according to claim 21, further comprising the step offorming a prescribed intermediate layer by epitaxial growth between saidlayer to be the first cell and said semiconductor substrate, whereinsaid step of separating said layer to be the first cell from saidsemiconductor substrate includes the step of removing said semiconductorsubstrate by etching and further removing said intermediate layer. 28.The method of manufacturing a compound solar battery according to claim21, further comprising the step of forming a prescribed intermediatelayer by epitaxial growth between said layer to be the first cell andsaid semiconductor substrate, wherein said step of separating said layerto be the first cell from said semiconductor substrate includes the stepof removing said intermediate layer by etching to detach saidsemiconductor substrate.